Hi,
@JoKalsbeek before diagnosis and even through my experimental stage to find which foods cause the worst reaction, I have had readings in the high teens, and when I was misdiagnosed with T2, my levels were high 20s.
It is because, when not controlled, and the very weak initial insulin response along with insulin resistance, glucose rises very quickly.
This does not mean that we cannot be termed non diabetic. It is non diabetic because normal readings are found when waking or fasting!
I am certain there are other conditions that are non diabetic, were high numbers are seen.
Keep safe